Nitrogen adsorption on thermally cleaned Si(100) surfaces by pure and plasma excited NH₃ is investigated by in situ IR absorption spectroscopy and ex-situ X-ray photoelectron spectroscopy with various temperatures from RT (25°C) to 800°C and with a treatment time of 5 min. The nitrogen coverage after the treatment varies according to the treatment temperature for both pure and plasma excited NH₃. In case of the pure NH₃, the nitrogen coverage is saturated as low as 0.13–0.25 mono layer (ML) while the growth of the nitride film commenced at 550°C. For the plasma excited NH₃, the saturation coverage was measured at 0.54 ML at RT and it remained unincreased from RT to 550°C. This indicates that the plasma excited NH₃ enhances the nitrogen adsorption near at RT. It is found that main species of N is Si₂= NH in case of the plasma excited NH₃ at RT while the pure NH₃ treatment gives rise to the Si–NH₂ passivation with Si–H at RT. We discuss the mechanism of the nitrogen adsorption on Si(100) surfaces with the plasma excited NH₃ in comparison with the study on the pure NH₃ treatment.